Pipette tips are used together with pipettes for metering liquids. Pipette tips are configured as small tubes with a relatively large aperture at one end for attaching to a pipette, a relatively small aperture at the other end for the passage of liquid and a through-channel between the large aperture and the small aperture. Pipettes have a receiving shaft for attaching the large aperture or a receiver for inserting one end of the pipette tip. They additionally have a gas displacement device which generally is configured as a piston and cylinder unit. The gas displacement device is attached via a through passage of the receiving shaft or the receiver to the large aperture of the pipette tip held releasably on the pipette.
An air column or another gas column is displaced by means of the gas displacement device, in order to aspirate liquid into the pipette tip and expel it therefrom. If the gas column is displaced away from the pipette tip, a specific amount of liquid is aspirated through the small aperture in the through-channel of the pipette tip. By displacing the gas column toward the pipette tip, an amount of liquid is dispensed from the through-channel through the small aperture.
Pipette tips are generally exchanged for fresh pipette tips after metering has been carried out, in order to prevent contamination of subsequently pipetted liquids by the remaining liquid. Single use pipette tips are generally made of plastics material.
When receiving and dispensing liquid, the finest droplets of liquid from the pipette tip can enter the pipette. With excessive gas displacement, large amounts of liquid can also be aspirated into the pipette through the large aperture. This liquid can enter the pipette tip during subsequent pipetting. As a result, this can lead to contamination of samples.
Filter pipette tips are already known for the prevention of contamination of the pipette. Said filter pipette tips have a porous filter filling up the cross-section of the through-channel. The filter is generally arranged in the vicinity of the large aperture. It is, for example, pressed into the through-channel of the small tube and/or supported on at least one projection in the through-channel. The region of the through-channel between the filter and small aperture serves to receive liquid. The filter retains aerosols and liquid in the filter pipette tip, so that they do not enter the pipette through the large aperture. Filters are, for example, configured as porous plugs made of plastics. They can be formed by sintering tiny plastics particles.
The barrier effect of known filters is insufficient for aerosols and liquids. Filters are already known with an additive which blocks the pores of the filter upon contact with the liquid. These filters are liquidtight but allow aerosols through. The additive can agglomerate and/or bleed out and contaminate a sample. Moreover, the frequently valuable liquid can be contaminated upon contact with the filter and/or is recovered with difficulty. The blocked filter thus does not allow the sample to be expelled.
A filter pipette tip is known from DE 36 35 598 C2 in which the filter is configured from a plurality of layers which are arranged on top of one another in a disc-like manner and have a progressive filter action, the permeability altering, in particular decreasing, from disc to disc. According to an embodiment, discs of different densities are layered on top of one another. With the increasing density, media which pass through repeatedly come into contact with wall portions of capillary passages. This can be utilised to initiate reactions but in turn also for increased filter action, namely when the suction or compression force reaches or exceeds a certain level. Thus the filter pipette tip has an increased loss of pressure which can result in metering inaccuracies or faulty operations.
A plurality of filter pipette tips is known with different characteristics. Thus, for receiving different metered quantities filter pipette tips are available in different sizes. Furthermore, filter pipette tips are known whose filters react in a particular manner upon contact with liquids, for example by obstruction or by altering their colour. Furthermore, filter pipette tips made of different materials and filter pipette tips with different levels of purity are known. Differentiating between the different filter pipette tips on the basis of their external appearance is problematic.
Proceeding therefrom, the object of the invention is to provide a filter pipette tip which has an improved barrier effect without impairing the pipetting. This object is achieved by a filter pipette tip with the features of each of claims 1, 5 and 18.
Moreover, a filter pipette tip is to be provided which facilitates the differentiation of filter pipette tips with other characteristics.